Field of the Invention
The present invention relates to a subscriber telephone set incorporating a first overvoltage protection means connected in the said set to the line inputs and also a second protection means provided up-stream of the hook contact intended to limit the power applied to an electronic transmission and dialing circuit associated with a push-button dial.
Although the introduction of electronic circuits in telecommunication equipment marked the beginning of much progress, the solution of certain safety problems among which is the protection against overvoltages, has become more difficult.
Actually, the construction proper of conventional electromechanical apparatus made them capable of resisting overvoltages of several tens, even several hundreds of volts, so that it was possible to use a simple and effective protection means such as spark arresters. This does not hold for equipment provided with electronic circuits which do not tolerate overvoltages which have been permissible so far.
This situation lead to the use of protection means, either active means formed by electronic circuits, or passive means formed by components having a non-linear characteristic.
The present-day protection circuits are not adequate to protect a subscriber set fully, as they cannot resist considerable overvoltages such as overvoltages which may be produced by, for example, inadvertent contact of a telephone line with an electric power mains cable.
The passive means such as voltage-dependent resistors (VDR) and positive temperature-coefficient resistors (PTC) render a less costly and effective protection possible. It is actually sufficient to provide at the input of the telephone set two PTC resistors arranged in series in each of the line conductors and a VDR in parallel with and down-stream to them. In the event of a line overvoltage the resistance of the VDR drops considerably, which causes a considerable current to circulate in the PTC whose warming-up increases the line impedance significantly.
However, the protection thus obtained is not complete in the case where the telephone set incorporates electronic circuits. It is known that the call procedure of a telephone set from the exchange is effected by applying a 50 Hz a.c. voltage of the order of 70 to 80 volts to the line. After the receiver has been lifted from the set the d.c. line voltage establishes itself at approximately 10 volts, which is the voltage from which the several electronic circuits of the telephone set are supplied.
If it is desired to maintain a normal operation of the ringing circuit the "knee" voltage of the VDR must not be lower than approximately one hundred volts, which means that an overvoltage below this value may be applied, in the "off-hook" condition of the set, to the electronic circuits of the subscriber set without the protection means responding.
If the electronic circuits are provided with internal supply voltage limiting and stabilizing means there is every risk that a prolonged overvoltage may cause the destruction of one or several circuits due to an excess current.
So as to satisfy the most severe requirements, two protection devices are employed at present, as mentioned in French Patent Specification No. 2132175, one of which limits the voltage at the line inputs of the telephone set, the other one being provided down-stream of the hand set to further reduce the power admitted to the electronic circuits which have for their function to process the transmission and dialing signals.
The fact remains that the apparatus such as it is described in the said Patent Specification may still be damaged when a prolonged overvoltage is applied to its terminals.
One of the objects of the invention is to protect a subscriber set from all possibilities of being put out of order or destruction resulting from an overvoltage of any value.
A further object of the invention is to make advantageous use of one of the embodiments of the protection device to effect the dialing procedure by periodically cutting-off the line current.
According to the invention, a telephone set incorporating a first overvoltage protection device connected in the said set, to the line inputs, also incorporating a second protection device provided downstream of the hand set, intended to limit the power applied to an electronic transmission and dialing circuit associated with a push-button dial, is characterized in that the said second protection device is in essence formed by a dipole electronic circuit arranged in series with one of the inputs of the said transmission and dialing circuit, by means of a dialing interrupter, this dipole circuit comprising a first transistor whose main current path is connected to one of the terminals of the dipole via a current measuring resistor, and to the other terminal of the dipole via a diode which forms together with a second transistor whose polarity is the reverse of the polarity of the first transistor, a current mirror supplying the control electrodes of the first transistor, while a third transistor is arranged in parallel with the control electrode of the first transistor which responds when the current flowing through the current measuring resistor reaches a predetermined value.
In accordance with an advantageous operating mode the dipole electronic circuit comprises a fourth transistor which acts on the control electrode of the first transistor to cut the latter off when the voltage at the terminals of the dipole circuit exceeds a predetermined threshold voltage.
Advantageously, the dipole power limiting electronic circuit has a current limiting auxiliary terminal connected to the transmission and dialing circuit. Alternatively, the dipole electronic circuit may also constitute the dialing interrupter, the line interrupting control terminal being connected to the control electrode of the first transistor by means of a diode which forms a current mirror together with a fifth transistor.
Thanks to its very short response time and to its particular characteristics, the second protection device ensures, in combination with the first device, complete safety against prolonged or short overvoltages, whether the set is in the on-hook or the off-hook condition. In addition, the construction of this second protection device is simple even for the case where it also performs the dialing function by periodical interruptions of the line current. It is therefore possible to integrate this circuit, either separately, or in the incorporated form in the integrated transmission/dialing circuit.
The following description given by way of example with reference to the accompanying drawings will make it better understood how the invention can be put into effect.